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Date:   Tue, 10 Jan 2023 00:55:51 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Zqiang <qiang1.zhang@intel.com>, quic_neeraju@quicinc.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check
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On Mon, Jan 09, 2023 at 11:32:26AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 09, 2023 at 12:09:48AM +0100, Frederic Weisbecker wrote:
> > On Sat, Jan 07, 2023 at 09:55:22PM -0500, Joel Fernandes wrote:
> > > 
> > > 
> > > > On Jan 7, 2023, at 9:48 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > 
> > > > ﻿
> > > >>> On Jan 7, 2023, at 5:11 PM, Frederic Weisbecker <frederic@kernel.org> wrote:
> > > >>> 
> > > >>> ﻿On Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:
> > > >>> (lost html content)
> > > > 
> > > > My problem is the iPhone wises up when I put a web link in an email. I want to look into smtp relays but then if I spent time on fixing that, I might not get time to learn from emails like these... 
> > > > 
> > > >> I can't find a place where the exp grace period sends an IPI to
> > > >> CPUs slow to report a QS. But anyway you really need the tick to poll
> > > >> periodically on the CPU to chase a quiescent state.
> > > > 
> > > > Ok.
> > > > 
> > > >> Now arguably it's probably only useful when CONFIG_PREEMPT_COUNT=y
> > > >> and rcu_exp_handler() has interrupted a preempt-disabled or bh-disabled
> > > >> section. Although rcu_exp_handler() sets TIF_RESCHED, which is handled
> > > >> by preempt_enable() and local_bh_enable() when CONFIG_PREEMPT=y.
> > > >> So probably it's only useful when CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n
> > > >> (and there is also PREEMPT_DYNAMIC to consider).
> > > > 
> > > > Makes sense. I think I was missing this use case and was going by the general design of exp grace periods.  I was incorrectly assuming the IPIs were being sent repeatedly for hold out CPUs, which is not the case I think. But that would another way to fix it?
> > > > 
> > > > But yeah I get your point, the first set of IPIs missed it, so we need the rescue-tick for long non-rcu_read_lock() implicit critical sections.. 
> > > > 
> > > >> If CONFIG_PREEMPT_COUNT=n, the tick can only report idle and user
> > > >> as QS, but those are already reported explicitly on ct_kernel_exit() ->
> > > >> rcu_preempt_deferred_qs().
> > > > 
> > > > Oh hmm, because that function is a NOOP for PREEMPT_COUNT=y and PREEMPT=n and will not report the deferred QS?  Maybe it should then. However I think the tick is still useful if after the preempt disabled section, will still did not exit the kernel.
> > > 
> > > I think meant I here, an atomic section (like bh or Irq disabled). There is no such thing as disabling preemption for CONFIG_PREEMPT=n. Or maybe I am confused again.  This RCU thing…
> > 
> > Right, so when CONFIG_PREEMPT_COUNT=n, there is no way for a tick to tell if the
> > the interrupted code is safely considered as a QS. That's because
> > preempt_disable() <-> preempt_enable() are no-ops so the whole kernel is
> > assumed non-preemptible, and therefore the whole kernel is a READ side critical
> > section, except for the explicit points reporting a QS.
> > 
> > The only exception is when the tick interrupts idle (or user with
> > nohz_full). But we already have an exp QS reported on idle (and user with
> > nohz_full) entry through ct_kernel_exit(), and that happens on all RCU_TREE
> > configs (PREEMPT or not). Therefore the tick doesn't appear to be helpful at
> > all on a nohz_full CPU with CONFIG_PREEMPT_COUNT=n.
> > 
> > I suggest we don't bother optimizing that case though...
> > 
> > To summarize:
> > 
> > 1) nohz_full && !CONFIG_PREEMPT_COUNT && !CONFIG_PREEMPT_RCU:
> >   Tick isn't helpful. It can only report idle/user QS, but that is
> >   already reported explicitly.
> > 
> > 2) nohz_full && CONFIG_PREEMPT_COUNT && !CONFIG_PREEMPT_RCU:
> >   Tick is very helpful because it can tell if the kernel is in
> >   a QS state.
> > 
> > 3) nohz_full && CONFIG_PREEMPT_RCU:
> >    Tick doesn't appear to be helpful because:
> >        - If the rcu_exp_handler() fires in an rcu_read_lock'ed section, then the
> >          exp QS is reported on rcu_read_unlock()
> >        - If the rcu_exp_handler() fires in a preempt/bh disabled section,
> >          TIF_RESCHED is forced which is handled on preempt/bh re-enablement,
> > 	 reporting a QS.
> >    
> >   
> > The case 2) is a niche, only useful for debugging. But anyway I'm not sure it's
> > worth changing/optimizing the current state. Might be worth add a comment
> > though.
> 
> Thank you both for the analysis!  I would welcome a comment.

I'm preparing that.

> One could argue that we should increase the delay before turning the
> tick on, but my experience is that expedited grace periods almost always
> complete in less than a jiffy, so there would almost never be any benefit
> in doing so.  But if some large NO_HZ_FULL system with long RCU readers
> starts having trouble with too-frequent tick enablement, that is one
> possible fix.

And last but not least: wait for anybody to complain before changing anything
;-))

Thanks.